Multiphase oscillators are known in the art. More specifically, RC oscillators, utilizing integrators and having adjustable resistance-capacitance values are known in the art. Such oscillators are typically used to generate user selectable, adjustable, frequency signals.
The multiphase oscillators of the prior art typically include circuitry for generating a number of signals, at a particular frequency and at a particular phase. In order to calibrate the amplitude of the output signal, a DC control voltage is applied to a voltage-controlled attenuator or other voltage controlled amplitude regulating device.
To obtain the DC voltage it is known to combine various phases of the generated signal. The various phases are typically combined by addition in a summing stage, to produce the output signal. The process of addition of the different phase signals, as well as other facets of signal generation in the prior art multiphase oscillators such as different methods for producing an appropriate loop gain, typically introduces harmonic distortion to the DC control signal derived from the various phases of the output signal.
There is thus a need in the prior art for methods or apparatus for reduction of the harmonic distortion in an output DC signal generated by combining phases of a variable frequency signal generated by an oscillator.
Prior art techniques for reduction of harmonic content of a DC signal are known.
In one such technique, a pair of squaring circuits is used in an amplitude detector to generate a cosine-squared and a sine-squared signal at a particular frequency. The resultant squared signals are summed to obtain a constant, DC, level having no AC component for comparison in a unity gain feedback circuit of a quadrature type oscillator.
However, such an approach, while theoretically useful, is impractical. The cost of multipliers (and squaring circuits) is substantial and thus mitigates against use of the sum-of-squares approach.
Accordingly, there is a need in the prior art for inexpensive, easily fabricated, practical circuits for reduction of harmonic content of DC signals generated from an oscillator output signal.
In another prior art approach, it is known to obtain the desired DC voltage by combining four phases of a signal, spaced apart by 90.degree., generated by a double integrating RC oscillator. In this approach, instead of using a half-wave rectifier to obtain the DC control signal, the four phases are summed by a circuit including four input diodes. The circuit provides an output whose amplitude at any one instant is the most positive of the four signals. Thus, the circuit functions to reduce the harmonic content of the DC control signal in comparison to earlier half-wave rectifier techniques for generating the control signal. However, the prior art, although successful in reducing some of the harmonic content of the control signal, provides output DC signals which contain ripple distortion at frequencies as low as the fourth harmonic. While reduction of the harmonic content by elimination of harmonics below the fourth is an advantage, the remaining harmonics nonetheless contribute to additional distortion at a level which is unacceptable for more precise applications.
The prior art thus fails to provide an economical, practical, circuit capable of successfully eliminating a large number of harmonics from a DC control signal, and thus of significantly reducing the total harmonic distortion of the signal to a level acceptable for high precision applications. There is thus a need for circuitry which simply and efficiently eliminates a large number of harmonics from a DC control signal, and accordingly eliminates substantial amounts of harmonic distortion from a control signal generated from oscillator signals.
More specifically, there is a need for practical circuitry capable of reducing or eliminating harmonic distortion in DC signals derived from an oscillator. Such circuitry is needed for removing harmonics at frequencies equal to or exceeding the fourth and higher harmonics of the fundamental generaled frequency and more specifically, where the DC signal is synthesized from a sum of N phases, for removing harmonics up to the 2Nth harmonic when N&gt;2.